1. Field of the Invention
This invention relates to an inkjet ink.
2. Description of the Related Art
As for the ink to be employed in a printer of an inkjet system, there has been proposed an ink employing pigments in order to improve printing qualities such as water resistance, light resistance, etc. In particular, an ultraviolet-curing ink (UV ink) which can be cured quickly and is minimal in volatilization of organic solvents and excellent in adhesion is now attracting much attention.
As for such a UV ink, one typical example is an ink comprising a radical polymerizable monomer, a photopolymerization initiator and a pigment. Further, there has been proposed a photopolymerizable UV ink comprising a cationic polymerizable monomer, a photo-cation generating agent and a pigment. Since an ink layer containing a UV ink is capable of instantly non-fluidized as it is irradiated with light, it is possible to reliably obtain printed matter of high quality. In an inkjet recording apparatus to be employed for such a quick drying printing, printing heads are linearly arrayed for accelerating printing and the printing is performed by running one pass on a printing face. Therefore, since ink discharging error leads to the generation of linear printing failure (missing) on the printing face, it is necessary to prevent such a discharging error as much as possible. Namely, the ink to be used for such an application as mentioned above is required to be excellent in stability of material in order to achieve very high printing accuracy and delivery stability.
A cation polymerizable ink is capable of overcoming the low sensitivity that may be caused due to the inhibition of curing to be caused by the poor adhesion of radical polymerizable ink and by the effect of oxygen. Since this ink is highly reactive, the physical properties such as viscosity thereof are inherently liable to fluctuate considerably and hence this ink is unstable. The reason for this may be ascribed to the facts that when active species generate once due to various factors (for example, heat, etc.), the active species cannot be easily deactivated, resulting in the generation of numerous dark reactions of ink.
Further, according to recent studies, it has been made clear that in order to enhance the stability of these inks, it is necessary to stabilize the conditions of inks not only macroscopically but also microscopically. For example, as the particle diameter of pigment becomes smaller and the dispersion stability of pigment becomes higher, the printing error of ink can be minimized more effectively. Not only the cation polymerizable ink mentioned above but also the ink containing therein an ionic substance (a polymerization initiator, a surfactant, etc.) is generally liable to flocculate with time due to salting-out and the like in a colloidal dispersion system thereof as compared with the conventional radical polymerizable ink. As a result, these inks are inferior in storage stability. When the viscosity, surface tension and elastic force of ink fluctuate due to the reaction of ink, it will lead to the disturbance of the ejecting shape of ink, to poor reproducibility of printing, and in the worst case, to a critical condition such as defective discharging of ink and clogging of ink. Therefore, this problem is very serious.
As for the criterion to determine the dispersion stability of pigment particles, although ζ(zeta)-potential has been conventionally employed some occasions, the employment of ζ-potential has been limited to the case where a dispersion medium is aqueous. In the case of UV ink where the aforementioned polymerizable monomer is employed however, there is no means other than adapting a method of confirming the dispersion stability through the investigation of actual changes with time of particle diameter of pigment and viscosity of ink.